A wide variety of spinal fixation systems exist. Some systems and their components will be discussed below, and are well known to those skilled in the art of orthopedic. These various systems are meant to provide a safe technique, a secure and stable implant of the components or constructs, and provide sufficient force to the spine to correct deformities, or aid in an healing process from trauma, or assist in degenerative conditions. In addition, these implants or constructs must resist fatigue and failure post operatively until bone fusion occurs.
The forces applied to the functional spinal unit include compression, tension, torsion, and shear. The motion associated with a spine is complex. Rotations and translations occur about three axes and result in six possible components of movement for any motion. It is therefore important that a spinal fixation unit for surgical implantation provide a high degree of stability.
An orthopedic surgeon typically employs the following protocol for installation of a spinal fixation device. The patient is positioned on an appropriate frame, such as an Andrew's frame or operating table, and is prepped and draped in the fashion standard for back surgery. The incision is made over the spinous process of the area to be decompressed. The incision is carried down through the dorsal lumbar fascia and the fascia is then incised down to the spinal lamina junction. Dissection is continued out to the tips of the transverse processes and is accomplished using the electrocautery and Cobb dissection tool. Self retaining retractors are then placed into the wound to allow clear visualization of the structures which have been denuded of their soft tissue. Further meticulous soft tissue dissection is performed with the removal of the supraspinous ligament and the interspinous ligament for the vertebral levels to be addressed in the surgery process. Intraoperative lateral x-ray confirms the position at the appropriate level, such as the thoracic, lumbar, or sacral levels. After implanting the spinal fixation device, the wound is then closed using standard operating procedures.
Use of pedicle screws is typical with instrumentation systems such as a Dynamic Transverse Traction (DTT) unit, the Steffee-VSP system (AcroMed Corporation), Isola Instrumentation (AcroMed Corporation, Cleveland, Ohio), or the like. Harrington devised the first universally accepted method of internal fixation for the treatment of spinal deformity. The current Harrington method uses stainless steel constructs. In 1974 Zielke developed a bone screw with a slotted head. In 1982 he used it for the first time through the pedicle of the vertebral arch, to correct kyphotic posture defects. These instrumentation devices are useful where lumbar segmental instability is a problem. Zielke's method, termed the VDS system, uses stainless steel.
Another system is the TSRH (Texas Scottish Rites Hospital) Spinal System, by Danek Medical, Inc. This system provides temporary stabilization until a solid spinal fusion develops. Use is indicated for such conditions as idiopathic scoliosis, neuromuscular scoliosis with associated paralysis or spasticity, spinal fractures, and neoplastic disease. Deficient posterior elements resulting from laminectomy or spina bifida could call for use of bone screws. Still another system uses the Galveston technique for pelvic fixation. A different approach to using bone screws is that of the MOSS-Titanium-mesh-cylinder system for spinal tumors between cervical vertebrae C3 and lumbar vertebrae L5. The MOSS-bonescrew-system is used for postural defects between thoracic vertebrae Th8 and sacrum S1. Other systems, such as the Dwyer system, use wire or cable as a securing device for the constructs. Wire tends to provide less stability for certain conditions. Different conditions require creating forces artificially to correct or maintain the spinal orientation. Posterior compression, anterior bone block, reposition by distraction, or anterior release are just some of the force applications for different conditions.
Pedicle screws, hooks, eyebolt assemblies, hex nuts, transverse rods, and cross-links are associated with these devices. Current pedicle screws have a yoke that has a u-shaped grove that conforms directly to a transverse rod. Both surfaces of the yoke are flush. The hex nut holds against the small yoke. However, hex nuts tend to loosen under the thousands of daily stresses experienced by the spine, unless they are securely fastened. This construct requires bending of the rod in order to conform to the lordotic (concave) or kyphotic (convex) curves in the surgical area. It is important to avoid excessive bending and rebending of these rods because fatigue resistance decreases as bending increases, leading to a more likely rod failure. Eyebolts can also score these rods, leading to earlier rod failure. On the other hand, some rods are too stiff, such as stainless steel rods, and do not lend themselves to contouring, although contouring is a positive characteristic in a rod.
Some existing washer-like spacers used between the rod and the bone screw provide for angular rotation of a pedicle or bone screw. However, they do not provide for height adjustment of the eyebolt assembly. Existing cross links have grooves in the center of rectangular ends on each cross link. This feature means that the essentially parallel rods must be exactly parallel or the cross link does not fit. Therefore other constructs are needed to stabilize the vertebral body area.
In certain areas of the spine, high stresses are created in the rods. Some systems avoid cross links in these areas because the stress would be too great and prevent stability from being achieved or being maintained. The links would fail. Contraindications occur with many devices for certain situations because of lack of their versatility for use in multiple circumstances.
Problems that exist in current spinal fixation systems include 1) lack of adequate strength in metallic rods, 2) lack of flexibility in the use of metallic rods, 3) severe stresses occurring within the constructs, 4) lack of height adjustment of the constructs, 5) lack of angular rotation capability within the bridge assemblies, 6) difficulty in locating and holding the bone screw with a forceps like device, and 7) difficulty in securing the eyebolt assembly into the yoke of the bone screw.
Thus, there is a continuing need for apparatus and methods for improving skeletal fixation systems, particularly in stabilizing spinal vertebrae under degenerative, trauma, or deformity conditions.